CORNING, Calif. -- Knowing when to apply nitrogen-based nutrients to trees can limit contaminants that seep into groundwater, experts advise.

A grower shouldn't put them on too early or too late, explained Patrick Brown, a plant sciences professor at the University of California-Davis.

Almond trees don't take up nitrogen while they're dormant, so growers need to wait until the trees are at full leaf-out to get maximum benefit from fertilizers, Brown said.

Then a tree's nutrient uptake diminishes as the summer passes. About 80 percent of the tree's uptake occurs within 120 days of bloom, Brown said.

"The reality is we have a lot of opportunity to think about nitrogen and how it gets into the tree," he told about 70 growers during a workshop here Dec. 5. "We can spend less money on nitrogen and have better trees."

Such thinking is becoming important for growers as new state scrutiny on nitrates in groundwater is anticipated. A UC-Davis study unveiled this spring showed that nitrate contamination of drinking water is a pervasive problem in the state's heartland and is bound to intensify in the coming years.

Already, farmers on the Central Coast are grappling with new application and monitoring rules. Gabriele Ludwig, associate director of environmental affairs for the Almond Board of California, warned growers here that similar rules could be imposed in the Central Valley.

"The fact is a lot of people in the Central Valley in California don't have access to clean water and state law says they shall," Brown said. "It doesn't matter who's to blame for all this."

Brown gave orchardists tips on how to make the best use of their applications of nitrogen, which can leach into groundwater as nitrates if not used by trees. Nitrogen is mobile in most soils, including those not thought of as permeable, he said.

UC scientists have been researching nitrogen uptake by almond and pistachio trees for the past five years. One thing they've found is the trees' uptake of the nutrient is proportional to their demand.

"They're kind of like us," he said. "We're not going to get smarter or more productive by eating two lunches."

Brown said producers should develop a nitrogen "budget" for their trees based on the anticipated crop size at the beginning of the season, and not be afraid to alter it as things change.

Generally, almond trees need about 68 pounds of nitrogen for every 1,000 pounds of nuts, he said. Growers can keep track of whether their orchards have enough of the nutrient by collecting leaf samples from at least 18 trees that are at least 30 yards apart.

"Your goal as a grower is to provide what is taken up by the tree," he said. "What really matters -- the big deal -- is how many nuts you have on your tree."

Online

UC Fruit and Nut Research and Information Center: fruitsandnuts.ucdavis.edu